Both varifocal and zoom lenses are built with movable elements that permit changing the effective focal length. A lens might be specified as 3.5-8mm or 6.5-52 mm, meaning the focal length can be adjusted within these ranges.
The key difference between a varifocal and a zoom lens can be explained by thinking about a lens that has been focused on an object at any focal length. A varifocal will need to be refocused whenever the focal length is adjusted; the zoom will stay in focus when the focal length is adjusted (i.e. when "zooming in" or "zooming out").
While this is an advantage in some applications, if a camera is going to be set-up once at an installation and then stay at the same focal length, the varifocal is a smaller lens that will do the job at a much lower cost than the zoom.
Another method for describing the range of focal lengths in a varifocal or zoom lens is the zoom ratio. This is ratio of the shortest focal length and longest focal length of the lens. So a lens could be specified as follows:
The method used to describe a lens is less important than is an understanding of the operating range of a specific lens (for example 3.5 – 8mm), and how the image is affected throughout this range.
